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8. INHIBITORS OF HEAT-SHOCK PROTEINS (HSP 90)

Protein folding is catalysed in vivo by isomerases and chaperone proteins. Molecular chaperones are ubiquitous proteins that assist folding, assembly, transport, and degradation of proteins within the cell. The first identified chaperones were heat-shock proteins (HSPs), whose names is derived from the elevated levels produced when cells are grown at higher-than-normal temperatures. HSPs stabilize other proteins during their synthesis and assist in protein folding by binding and releasing unfolded or misfolded proteins using an ATP-independent mechanism. Proteins unable to maintain their proper shape are broken down by the proteasome (see Section 1 of Chapter 10) and eliminated, as shown in Fig. 9.33. These events may be favourable if the proteins are previously mutated and hence dangerous for the survival of the cell, but they become a problem if the proteins are necessary for its normal functioning.

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HSP90 and other cha perones

FIGURE 9.33 Function of heat-shock proteins.

HSP 90 is the best known of HSPs and its activity is coupled to an ATPase cycle that is controlled by several cofactors. It has three major domains, namely a highly conserved N-terminal ATPase domain, a middle domain, and a C-terminal dimer-ization domain. The crystal structure of HSP 90 bound to ATP has shown how this nucleotide is hydrolysed,135 but the detailed mechanism of protein folding remains unknown.

HSP 90 has emerged as an attractive cancer target because its inhibition blocks a large number of cancer-related signalling pathways since a large number of intra-cellular signalling molecules require association with HSP 90 to achieve their active conformation, correct cellular location, and stability.136 These include steroid hormone receptors, transcription factors like the tumor suppressor protein p53 and kinases like Src-kinase.

The conformational changes that take place in HSP 90 after binding and hydrolysis of ATP regulate the stabilization and maturation of client proteins, including hypoxia-inducible factor-1 (HIF-1), a relevant anticancer target.137 This ATP site is known by X-ray crystallography to be very different from that of kinases, allowing the design of inhibitors with high selectivity with regard to other ATP-binding proteins.

The design and study of selective inhibitors of HSP 90 was initially controversial because this protein is critical for the survival of both normal and sick cells. However, HSP does not have much activity under normal conditions. When the cell is under stress by genetic mutations or environmental changes such as heat or infection HSP 90 activity is increased as an emergency response that stabilizes partially unfolded proteins and helps them to achieve their correct shape. This activity also assists the survival of cancer cells despite an abundance of misfolded

FIGURE 9.33 Function of heat-shock proteins.

and unstable proteins, and this is one of the reasons to study HSP 90 as an anticancer target.

The main strategy employed in the design of HSP 90 inhibitors is based in the synthesis of analogues of the natural antitumor geldanamycin, a benzoquinone derivative belonging to the ansamycin class, although some companies working in this field are designing entirely synthetic molecules not related to this compound.

Geldanamycin was originally believed to be a TK inhibitor, but it was later identified as an ATP-competitive inhibitor of HSP 90. It could not be advanced to the clinical stage because it showed unacceptable hepatotoxicity, probably associated with the presence of the electrophilic methoxybenzoquinone moiety. For this reason, displacement of the 17-methoxy group by nucleophiles led to less toxic analogues such as tanespimycin (17-allylaminogeldanamycin, 17-AAG).138 Another problem associated with geldanamycin is its very low solubility, which was solved with the development of the water-soluble analogue alvespimycin (17-dimethylaminoethylaminogeldanamycin, 17-DMAG).139 Both analogues were better tolerated than the parent natural product and are under clinical trials. In another approach, the problematic quinone moiety of 17-AAG was reduced to the hydroquinone stage. The resulting compound, IPI-504, can be formulated as a soluble salt that is suitable for intravenous or oral formulations. It has shown encouraging results in Phase I trials in patients with gastrointestinal stromal tumors that were resistant to imatinib, although further clinical development is necessary.

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